Windows Defender Update Error

Windows Defender Update Error normally has error code 8020000D. To fix this problem follow the below steps. 

Some other program like chkdsk is currently running which locked the disk and so BITS is not able to write to the disk.

If the issue is not being caused by a scheduled check disk, then "something" else is locking the disk or preventing BITS from writing to the download location. That "something" could be security software [antivirus/3rd party firewall/etc], malware, or the Hard Drive may have issues.

If this is Windows Vista, open the Services console.

• *Right* click Computer either on the Desktop or Start Menu and 
• choose 'Manage'; 
• allow the User Prompt; 
• open Services and Applications; 
• then Services. 
• click on Background Intelligent Transfer Service and see if you are able to Start it. 
• Then try to update Defender again.

Alternatively, you can manually download the latest Defender definition from here, save it, and then manually install it:

http://www.microsoft.com/security/portal/Definitions/ADL.aspx

Windows Vista Error Code 737D